The present invention relates to a device or craft for selectively picking up a layer of light liquid, such as hydrocarbon, floating on the surface of a sheet of water likely to be subjected to a swell, usable more especially for cleansing zones covered with a layer of hydrocarbon following accidental discharges.
It should first of all be recalled what the problems are which are met with in the construction of a cleansing or depolluting device for use at sea.
Polluting hydrocarbon is in the form of a thin layer (of the order of a millimeter) formed by a hydrocarbon phase which may be very viscous due to evaporation of the light components or by a polyphase emulsion of hydrocarbon with sea water and/or air, following the stirring caused by the waves.
The device must be designed for cleansing as large a width as possible at each passage. The water inevitably picked up at the same time as the pollutant must represent as small a fraction as possible of the part extracted and stored. To obtain this latter result is thwarted by the very small ratio between the thickness of the pollutant layer and that of the water layer which must necessarily be picked up because of the variations in level due in particular to the swell.
Patent publication No. FR-A-2 467 769 describes a device of a type which comprises a hull provided with propulsion means for running before the sea, the hull having a central part projecting forwardly with respect to the two lateral parts which define with the central part ducts leading to separators and the central part having deflector means such as fins for creating swirls whose orientation tends to reduce the divergence of the surface flow about the hull.
The deflector means allow a double result to be reached. On the one hand, the device thus sweeps the sea between two current lines which, upstream, have a much greater separation than they would have had in the absence of these means; correspondingly, there is a thickening of the light liquid layer at the intake of the delivery ducts. Because the pick-up operation takes place running before the sea, the wake of the ship causes damping of the swell.
Thus, thicknening by a factor of the order of 2.5 may be achieved between the open sea upstream of the device and the water intakes of ducts 16. In practice, because of the convergence of the current lines, a thickening rate of the hydrocarbon layer of the order of 2.5 may be achieved.
Such thickening remains however very insufficient for feeding a separator, more especially of the centrifugal type, under conditions such that the ratio between pollutant and water in the flow picked up is acceptable.
The invention aims more particularly at providing a pick-up device in which progressive thickening of the light liquid layer is achieved along an open stream flow until separation by a free surface swirling procedure, with vertical axis and central pick-up.
This thickening must be carried out very progressively from the stem of the central part as far as the pick-up by an axial plunger tube. In fact, the equilibrium of a light liquid layer of variable thickness causes this liquid to be carried along by the water resulting from a speed discontinuity at the interface with a tangential constraint proportional to the gradient of the square of the thickness; this equilibrium becomes unstable beyond a limit value of this constraint, so also of this gradient or of the speed discontinuity.
To this end, the invention proposes a device of the above-defined type, characterized in that each duct or passage comprises a floor with deviating profile whose leading edge projects forwardly of the ducts, for slowing down the flow upstream of the leading edge and causing progressive thickening of the layer, and in that it is separated by an approximately vertical dividing wall over a part of its height extending at a distance from a horizontal limit of the water layer in two sub-channels one of which communicates upstream with the inlet of the duct and, downstream, with the separator, and the other of which, separated from the first by the dividing wall, opens upstream into means for pumping and discharging to the rear of the device.
The dividing wall is advantageously expanded at its lower part, situated at a distance from the floor, for limiting the curvature of the paths of the liquid which passes from one sub-channel to the other; the total section of the two sub-channels is substantially constant in the flow direction, the section of the first sub-channel being reduced whereas the other increases.
Thus, thickening is achieved by three successive phenomena in a free surface stream:
a free swirling with horizontal axis approximately parallel to the plane of symmetry of the ship, and generated by the deflector means, typically by means of an immersed aerofoil surface connected to the bottom of the central part of the hull and extending forwardly beyond the stem. This swirling superimposes, in the divergent speed zone generated by the central part, speed components converging towards the plane of symmetry at the level of the free surface and causes progressive thickening of the layer by constant speed convergence (rate of thickening of the order of 2.5);
an oblique and approximately horizontal swirl due to the profiled deviating floor forming the bottom of each duct and whose leading edge has a negative camber, at least in the vicinity of the central part of the hull. The swirl induces on the surface speed components which progressively slow down the flow upstream of the leading edge of the floor at the cost of a very moderate divergence. The result is slowing down of the layer, with almost constant width, and a new progressive thickening (rate of the order of 2);
drawing off of the major fraction of the flow picked up in the duct above the leading edge of the floor and which remains completely separate from the flow outside the device until it is finally ejected after passing through a pump, this configuration allowing better control of the progressive drawing off of practically the whole of the flow. For that, practically from the intake of the duct and as far as injection into the swirl of the separator, the duct is separated into two sub-channels by the substantially vertical dividing wall oblique with respect to the median plane of the device, which intersects the floating line and goes down to a short distance from the floor. Since the sum of the areas of the cross sections of the two sub-channels remains substantially constant, that of the discharge (or drawing off) sub-channel increases from 0 to about 9/10 of the total area, whereas that of the supply sub-channel which receives initially the whole flow picked up is correspondingly reduced. Thus, the speed component parallel to the plane of symmetry remains almost constant in the supply sub-channel, both for the water and for the pollutant layer which is progressively thickening (rate of thickening of the order of 10). This arrangement may moreover be reversed, the supply sub-channel being supplied over the dividing wall which forms and overfall.
The floor of the passage is advantageously formed by a thick dividing wall whose sections through vertical planes parallel to the plane of symmetry of the hull are deviating profiles having then a very convex lower face. Usually, the lower face goes down approximately as far as a depth equal to the ship's draft to then rise again towards the rear. The upper face will then have a horizontal sill parallel to the leading edge and immersed at a depth equal approximately to a third of the ship's draft and, rearwardly of this sill, will go down as far as the maximum depth compatible with the thickness required for the mechanical resistance of the floor. The initial increase of the depth of the duct simplifies the problem posed by the loss of effective cross section due to the dividing wall separating the two sub-channels since it compensates for this loss by an increase in the total available cross-section.
At the rear, the vertical dividing wall joins the lateral inner wall of the supply sub-channel.
The leading edge of the dividing wall separates permanently the supply sub-channel, which then plays the role of injection gutter for the open stream swirl of the separator, from the discharge sub-channel which becomes a simple exhaust sub-channel from which the whole flow is drawn off by a pump which may form a means for propelling the device.
The formation in the twin phase flow of breakers or sudden movements must be avoided. This problem is made particularly acute because the device must be designed for working in troubled waters, for example subjected to a swell or a windswept sea. The conditions to be fulfilled, in the case of a permanent flow, for avoiding passing from a torrential flow to a fluvial flow which gives rise to jumps are already known. In the case of periodic working and even when the flow has parts under load where the periodic forces cause periodic compression of the waves, theoretical considerations bring out the conditions to be fulfilled for avoiding breakers in the twin phase flow and the stirring which it causes.
Experience on models has confirmed these considerations which were in no wise evident for a man skilled in the art, used to considering only permanent flows. They lead to providing damping of the swell coming from the rear when running before the sea by a rear immersed fin which provides unexpectedly additional favorable properties. Moreover, it is desirable to interpose, in the discharge sub-channel downstream of the dividing wall in the general flow direction, an immersed overfall which causes locally a torrential flow, fluvial flow transition and the formation of a sudden movement and breakers at a well-defined location where this phenomenon presents no disadvantage and avoids the formation of new breakers further upstream.
In a particular embodiment, the sub-channel of the device which opens into the pumping means presents successively a fraction forming a damping basin, then a part under load opening into the pumping means. The sub-channel has a full wall, so that the pumps can only draw water which has penetrated into the sub-channel by passing over or under the dividing wall.
Since the flow sucked in by the pumping means is substantially constant, or at least varies little, whereas the flow picked up by each supply duct varies periodically, for example because of the swell, the volume of water contained in the basin limited upstream by the dividing wall varies as a function of time. The low level of the free surface in the basin must not drop below a minimum value for which the pumping means would draw in air, which would risk damaging them. This result may be reached by giving the basin large dimensions, as seen from above. But, when the device is provided for operation in swells with a large trough, inacceptable dimensions of the basin, as seen from above, are reached.
To allow operation in conditions such that the flow picked up varies appreciably, without for all that requiring a large-sized damping basin, the sub-channel which opens into the pumping means is advantageously provided with means for supplying it with water from the water layer, for supplying a make-up flow to the pumping means, at least when the level in this channel drops below a given level or exceeds it.
These means for supplying a make-up flow may be limited to an opening communicating with the water layer provided in the wall of the sub-channel, advantageously in the floor. This opening will be generally placed at the inlet of the under load part of the sub-channel which opens into the pumping means or immediately upstream. The input of the part under load may form a downward projecting sill with respect to the downstream portion, so as to better avoid the intake of air towards the pumping means.
The invention will be better understood from reading the following description of particular embodiments given by way of non-limiting examples.